The invention relates to a system for mobile communication, comprising at least one mobile station and a communication domain having at least one communication station, which communication domain overlaps another communication domain at least partially. Such a system is known in practice.
In systems for mobile communication it is possible to set up, within a certain communication domain, a communication link between a mobile station and one or more communication stations of said communication domain. In this respect, such a communication domain, hereinafter also referred to as domain, may comprise a country, a region or, for example, only a factory site. The extent of a communication domain is determined by the range of the communication stations present in said domain. Said communication stations may comprise so-called base stations, but may also be constituted by e.g. satellites. In this text, the term base station will be used in the most general sense, i.e. meaning communication station in general and including satellites and other communication means.
Within a domain, mobile communication in two directions is, in principle, possible for a user registered therein via the base stations of said domain. In this connection, on the one hand, a mobile station can contact a base station in order to set up a link in this way and, on the other hand, a base station can contact a mobile station located in the domain concerned and registered therein. Registration of a (mobile) station in a particular domain thus allows (the network of) that domain to reach said station and vice versa. Registration, in this context, therefore comprises not only the notification (registration proper) allowing a domain to reach a mobile station, but also the recording of the availability of a domain in a mobile station, thus allowing a mobile station to reach a domain.
In practice, different domains will coincide completely or partially and therefore overlap. Thus, in regions in the vicinity of country borders, domains of the different countries will have a certain overlap, and the relatively small communication domain of a factory site may fall within the communication domain of the country concerned. In the case of overlapping communication domains, however, a number of problems arise with existing systems for mobile communication, as will be explained in greater detail below.
Within the overlapping areas, it is possible, in most cases, for a mobile station to communicate with any desired domain of the overlapping domains. If the mobile station is communicating with one of the domains or is at least registered therein, communication with and/or registration in the remaining domains is, however, no longer possible and said station has become inaccessible for said remaining domains. It will be clear that this is undesirable in many cases.
Usually a mobile station is assigned to a single user. The registration of the mobile station in a domain then has the result that said one user can be accessed by other users on said mobile station and, for his part, he can reach others. In some systems for mobile communication, however, it is possible to register a plurality of users with the aid of one mobile station. As a result, said users can all be reached on the respective mobile station registered, for its part, in the domain and they can reach other users from there. In this case, however, the problem arises that the mobile station is registered in only one domain and that the users of said mobile station can therefore only access one domain at a particular instant. Although it is therefore possible in existing systems to register a plurality of users via one mobile station, it is not possible to register said users simultaneously in different domains.
Prior Art techniques, as described in the references below, do not offer a solution to the above-mentioned problems.
WO-A-93 16549 discloses a communication system in which a mobile station can place and receive telephone calls in both a cordless and a cellular telephone system. The mobile station has a separate telephone number for each system, call routing priority being established for the relative numbers. An incoming call will first be routed to the system associated with the respective number and then, if necessary, be re-routed. This may result in the so-called "tromboning effect", causing an inefficient use of the network facilities. Furthermore, this known system does not provide for multiple users of a single mobile station. Moreover, the "domains" disclosed in this prior art reference have different technologies, i.e. cordless and cellular, while the mobile station has a different telephone number for each technology (domain). Also, the mobile station of said reference does not support multiple users and/or services.
U.S. Pat. No. 5,003,629 discloses the use of a single subscriber identification for a plurality of RF communication systems. A centralized database is provided for converting system-dependent identifications into universal identifications. This has the disadvantage that for large numbers of mobile stations and/or subscribers a very large database is required. In case several databases are used, the identifiers may differ between databases, thus counteracting their uniqueness. Moreover, this known system does not allow for multiple users and services being supported by a single terminal.
WO-A-92 19078 discloses a method in mobile telephone systems in which a subscriber identity module (SIM) is allocated at least two identities, one of which is activated by the user. The two or more identities pertain to business and private calls, or to different potential users, but not to different domains. Only one identity can be used at a time in this prior art technique.
NL-A-93 01494 describes a system for mobile communication arranged for selecting an available communication domain on the basis of a preference list.
WO-A-92 08326 discloses a method for a communication unit to operate in either a trunking communication system or a cellular communication system, where the trunking and cellular systems overlap. This known method comprises the monitoring of the control channel of the other system in order to change affiliation. However, only one affiliation at a time can be supported.